Embodiments of the invention relate generally to a nitride semiconductor device.
Switching devices for use in power control are required to have a low ON resistance and a high breakdown voltage. On the other hand, semiconductor switching devices exhibit a trade-off, which decreasing the ON resistance causes a decrease in breakdown voltage, and have a breakdown voltage limit unique to the semiconductor material to be used to a desired ON resistance.
Since nitride semiconductors such as GaN have a larger band gap than Si, the trade-off of the ON resistance and the breakdown voltage unique to the material can be drastically improved. Therefore, switching devices made of a nitride semiconductor are expected to achieve a high breakdown voltage at a low ON resistance as compared with conventional switching devices made of Si. A field effect transistor using heterojunction (HFET: heterojunction field effect transistor) of n-AlGaN and i-GaN and a Schottky barrier diode (SBD) using GaN are regarded as promising switching devices made of a nitride semiconductor. (JP-A 2007-180143 (Kokai); Japanese Journal of Applied Physics Vol. 44, No. 9A, 2005, pp. 6385-6388; and Proceeding of 2004 International Symposium on Power Semiconductor Devices & ICs, pp. 319-322)
On the other hand, as well as resisting to application of a high voltage, devices used for power control are required to protect passive devices, other switching devices, and the like in a circuit by clamping a voltage in order to prevent the voltage applied to subsequent stage circuits from becoming excessively large in the case where the applied voltage is overlapped with noise and becomes an overvoltage. In other words, passing a certain level of current to clamp the voltage against the overvoltage is an essential function.
However, in switching devices made of a nitride semiconductor, when a high voltage is applied to cause avalanche breakdown, a trouble occurs in which an overcurrent readily flows to lead to destruction of the device. That is, there has been the problem that a voltage clamp function utilizing a current generated by avalanche breakdown cannot be provided.